Iii-nitride light-emitting devices with reflective engineered growth templates and manufacturing method

ABSTRACT

A light emitter includes a first mirror that is an epitaxially grown metal mirror, a second mirror, and an active region that is epitaxially grown such that the active region is positioned at or close to, at least, one antinode between the first mirror and the second mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of and claims benefit of U.S.patent application Ser. No. 13/075,104 filed Mar. 29, 2011, titled“III-Nitride light-emitting devices with reflective engineered growthtemplates and methods of manufacture” (to issue as U.S. Pat. No.8,253,157 on Aug. 28, 2012), which is a divisional of and claims benefitof U.S. patent application Ser. No. 11/882,730 filed Aug. 3, 2007,titled “III-Nitride light-emitting devices with one or more resonancereflectors and reflective engineered growth templates for such devices,and methods” (which issued as U.S. Pat. No. 7,915,624 on Mar. 29, 2011),which claimed the benefit under 35 U.S.C. Section 119(e) of thefollowing U.S. provisional patent applications:

-   -   U.S. Provisional Patent Application No. 60/835,934, filed on        Aug. 6, 2006, by R. J. Jorgenson, entitled “III-Nitride        light-emitting devices with one or more resonance reflectors and        reflective engineered growth templates for such devices, and        methods”; and    -   U.S. Provisional Patent Application No. 60/821,588, filed on        Aug. 7, 2006, by R. J. Jorgenson, entitled “III-Nitride        light-emitting devices with one or more resonance reflectors and        reflective engineered growth templates for such devices, and        methods”; each of which non-provisional and provisional patent        applications is incorporated herein by reference in its        entirety.

BACKGROUND

1. Field of Invention

The invention is related to semiconductor light emitters.

2. Description of Related Art

Throughout this application, references are cited. The respectivedisclosure of each of these references is incorporated in its entiretyby reference.

Light emitting diodes (LEDs) are semiconductor devices that generatelight from electrical excitation where electrons and holes combine toannihilate, and thereby forming photons.

These structures are typically grown on sapphire or silicon carbidesubstrates by OMVPE (Organo-Metalic Vapor Phase Epitaxy).

FIG. 1 shows one example of an OMVPE grown standard group III-nitridesemiconductor LED that comprises of a sapphire substrate (101), anintrinsically doped gallium nitride (GaN) buffer layer 2 μm thick (102),a silicon doped 2 μm n-type GaN layer (103), an Indium Gallium Nitride(InGaN) active region (104) comprised of a single quantum well ormultiple quantum wells, a current blocking layer (105) comprising ofmagnesium doped p-type AlGaN, and a magnesium doped p-type GaN layer(106).

This LED structure is epitaxially grown on a substrate, which in thiscase is sapphire, such that several LEDs are formed on the surface ofthe substrate and electrical terminals (207) (208) are positioned on then-type GaN layer (203) and the p-type GaN layer (206) of each single LEDas shown in FIG. 2.

Group III-nitride LEDs require a thick GaN buffer layer (102) of about 2μm when grown on a nonconductive sapphire substrate as described in U.S.Pat. No. 4,855,249 (Isamu Akasaki et al., Aug. 8, 1989) and U.S. Pat.No. 5,686,738 (Theodore D. Moustakas, Nov. 11, 1997). This is to achievedevice quality material before the n-type layer (103), active region(104) and p-type layers (105) (106) of the device are grown. Althoughthere are other methods, generally this extended 2 μm thickness isdesirable to allow the GaN buffer (102) to coalesce during growthresulting in device quality material.

Standard group III-nitride semiconductor LEDs usually have low lightextraction due to the refractive index contrast between thesemiconductor (N_(GaN)=2.4) and air (N_(air)=1). Most of the lightemitted inside an LED is unable to escape through Snell's window toreach the outside medium (air), and thus has about 6% extractionefficiency from the extraction surface of the LED.

One method to improve light extraction may involve shaping the lightexiting surface of the device to reduce the amount of generated lightthat is lost to total internal reflection as described in U.S. Pat. No.5,779,924 (Michael R. Krames et al., Jul. 14, 1998). A shaping techniquethat improves light extraction comprises random texturing of the devicesurface to achieve light scattering.

Another light extraction enhancement approach is to form a layer with aphotonic crystal structure as described in U.S. Pat. No. 6,831,302(Alexei A. Erchak et al., Dec. 14, 2004). If designed accordingly, aphotonic crystal may inhibit guided modes so that more light isextracted through vertical modes or direct guided modes out of thedevice by diffraction. Surface texturing and photonic crystal structuressuffer from added complexity due to extra techniques and processingsteps which may include extra layer formation or etching steps.

As shown in FIG. 3, some III-nitride LEDs utilize a metal mirror contact(308) on the p-type GaN layer (306) side of the device as described inU.S. Pat. No. 6,573,537 (Daniel A. Steigerwald et al., Jun. 3, 2003).The metal mirror contact is deposited after the epitaxial process. Inthis example the metal mirror contact (308) covers the entire p-GaNlayer (306). Adjusting the p-GaN layer thickness and as a resultpositioning of the mirror allows the device to utilize optical cavity(309) effects as seen in FIG. 3 and FIG. 4. Light emitted from theactive region (304) self interferes due to reflection from the closelyplaced metal mirror (308). Optical cavity effects may increase the lightemission into the vertical modes while reducing the total number ofhorizontal optical modes. In this example, vertical modes are readilyextracted through a transparent substrate though Snell's window.Positioning the center of the active region (304) of the LED at, orclose to, a maximum (402) of the optical field distribution (401) asshown in FIG. 4, assists in light extraction. The local maximums (402)of the optical field distribution (401) are referred to as antinodes(402) because they are antinodes of the standing optical wave.Generally, positioning the center of the active region (304) at or closeto the closest antinodes (402) to the metal contact (308) is desired toallow more light through Snell's window. As shown in FIG. 4, antinodes(402), of the standing optical wave (401), are positioned periodicallyaway from the metal contact (308).

The above mentioned light extraction approaches are more easily appliedto the p-type GaN layer side of the device which is readily accessiblefor further processing after the epitaxial crystal growth is completed.Because the sapphire substrate is nonconductive and it is preferred togrow the conductive n-type GaN layer side first, the n-type GaN layerside becomes buried and difficult to access from the substrate side forfurther processing due to the extreme hardness of sapphire.

Dual-mirrored resonant cavity light emitting diodes (RCLEDs) ormicrocavity light emitting diodes (MCLEDs) represent a further method ofincreasing light extraction from a semiconductor light emitting device.The active region is located within an LED in such a way as to create anoptical cavity between two properly placed mirrors that direct lightemission into vertical modes or a single mode by reducing the totalnumber of optical modes within an LED.

Coupling a highly reflective mirror with a partially reflective mirrorto create a cavity has been predicted to increase light extractionefficiencies in the 30% to 50% range over standard LEDs as mentionedwithin U.S. Pat. No. 6,969,874 (Gee et al., Nov. 29, 2005).

The key to a properly functioning cavity LED is the placement of themirrors relative to the active region to obtain resonance andconstructive interference.

Coupling a metal mirror on the outer p-type III-nitride surface of a LEDto an active region placed at or close to an antinode of a standingoptical wave can be performed relatively easily with standard depositiontechniques as described in U.S. Pat. No. 6,573,537 (Daniel A.Steigerwald et al., Jun. 3, 2003).

One technique to couple a mirror within the n-type III-nitride region ator close to an antinode of an standing optical wave includes theepitaxial growth of Distributed Bragg Reflectors (DBRs) composed ofalternating layers of semiconductor materials, each with differentrefractive indexes and quarter wavelength thicknesses. A large number ofthese layers may be required to achieve sufficient reflectivity for theoptical cavity.

While these DBRs may be grown epitaxially, they have a number ofinherent disadvantages. The alternating material layers often sufferfrom a lattice mismatch that may lead to increased wafer cracking,poorer crystal quality, reduced yield, lower uniformity, and highermanufacturing cost. Moreover, DBRs are more electrically resistive,compared to metal or other semiconductor materials, resulting in poorcurrent injection into the device.

U.S. Pat. No. 6,969,874 (Gee et al., Nov. 29, 2005) discloses aFlip-Chip Light Emitting Diode with a Resonant Optical Microcavity usinga DBR at or close to an antinode of a standing optical wave. The DBRspecified for the device uses better lattice matched materials andrequires fewer alternating layers compared to previous DBRconfigurations. Nevertheless, while potentially an improvement overprevious DBRs, the device has not adequately solved the manufacturingcomplexities or conductivity shortcomings inherent in DBR materialcomposition.

Another method to place a mirror within the n-type III-nitride regioncomprises of removing the base substrate and any buffer layers, followedby a thinning and polishing of the n-type III-nitride layer in such away as to create an interfacial mirror located optimally for themicrocavity.

U.S. Patent Application Publication 2007/0096127 (P. Morgan Pattison,May 3, 2007) discloses a MCLED with an interfacial mirror on the n-typeside of a III-nitride device coupled with a metal mirror deposited onthe p-type III-nitride side of the device and an enclosed active regionplaced at or close to an antinode of an standing optical wave betweenthe two reflective surfaces.

Fabrication of this MCLED requires laser-lift-off, as described in U.S.Pat. No. 6,071,795 (Nathan W. Cheung et al., Jun. 6, 2000), to removethe substrate. Additionally the n-type III-nitride layer must be etchedto a precise and accurate thickness to create an optimally positionedinterfacial mirror relative to the active region and a highly reflectivemetal mirror deposited on the p-type III-nitride surface of the device.

While this approach has been shown to function, the process oflaser-lift-off and subsequent etching is difficult to commercialize andto obtain high yields.

What is desired is a microcavity LED structure that does not requirecumbersome material removal or complicated layering. Further it isdesirable to combine various light extraction structures as describedabove while allowing for high current injection.

SUMMARY

Device structures utilizing optical cavity effects for enhanced lightextraction comprising of an active region of an optimized thickness andplacement with respect to light extraction features and with respect tovarious configurations of an epitaxially grown metal mirror isdescribed. For purposes of this disclosure, these embedded mirrors (ormirror configurations) are defined as Grown-Epitaxial-Metal-Mirror(GEMM), or the GEMM layer.

In some examples, device structures may comprise an optical cavity thatcomprises one GEMM and a second mirror. The second mirror may be aDistributed Bragg Reflector (DBR), non-epitaxially grown metal mirror,interfacial mirror, a GEMM, or any other mirror structure. In otherexamples, said second mirror may be substituted with a roughenedsurface, a photonic crystal, or other light extraction structures. Anycombination of light extraction structures may be coupled with the GEMM.There are many variations that may be employed including combiningmultiple light extraction structures on one or both sides.

In some configurations, light may propagate through the top of thedevice, away from the substrate side. In other configurations, light maypropagate through a transparent substrate at the bottom of the device inthe form of a “Flip Chip” MCLED or RCLED. In other configurations thelight may exit more than one direction.

To create a resonant cavity device, the GEMM may be placed at or closeto a standing optical wave node to create constructive interferencebetween the light generated by the active region and the reflected lightfrom the GEMM. This enables light to be directed into modes (or a singlemode) to increase the amount of light propagating out of the LED. Lightgenerated by optical cavity devices are more directional and spectrallypure compared to standard LEDs.

The GEMM may be grown fully non-transparent and highly reflective as perits bulk material properties or grown semi-transparent to fit therequirements of the different device structures by simply adjusting theGEMM growth time, and thus thickness during epitaxy. The materials usedfor the GEMM closely match the lattice constant of the III-nitridelayers and are grown of device quality.

Appropriately utilized GEMM layers have a number of advantages overcurrent appropriately utilized device structure components likeDistributed Bragg Reflectors (DBRs) or non-epitaxial mirrors.

For example, GEMM layers may not require the layering of numerousalternating semiconductor materials, like DBRs, that may create defectsand cracks.

Embedding a planar specular GEMM layer within a light emitting devicestructure, such that optical cavity structures described herein areformed, may not involve material removal such as laser-lift-off ormaterial etching. The steps to create these alternative structures withthe GEMM are less process intensive and more easily commercializedcompared to other structures described above.

Additionally, the GEMM layer may be used as a conductive layer toincrease electrical current injection and current distribution.Furthermore, the superior current distribution may improve electrostaticdischarge reliability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an epitaxial stack of a standard III-nitride LED;

FIG. 2 illustrates a standard III-nitride LED after processing;

FIG. 3 illustrates a standard III-nitride LED with a non-epitaxial metalmirror contact (308) for enhanced light extraction;

FIG. 4 illustrates a conduction and valence band diagram of standardIII-nitride LED with a non-epitaxial metal mirror contact for enhancedlight extraction with optical field approximated;

FIG. 5 illustrates an epitaxial stack of a III-nitride LED with GEMMaccording to the first, second, third and fourth embodiments of theinvention;

FIG. 6 illustrates a III-nitride Resonant Cavity LED (also known asmicrocavity LED) with GEMM and a non-epitaxial metal mirror for enhancedlight extraction according to the first embodiment of the invention;

FIG. 7 illustrates a conduction and valence band diagram of the LED inFIG. 6 with optical field approximated;

FIG. 8 illustrates a III-nitride resonant cavity LED with a GEMM and anonepitaxial transparent conductor for enhanced light extractionaccording to the second embodiment of the invention;

FIG. 9 illustrates a conduction and valence band diagram of the LED inFIG. 8 with optical field approximated;

FIG. 10 illustrates a III-nitride surface roughened assisted resonantcavity LED with a GEMM coupled to a roughened p-GaN layer andnon-epitaxial transparent conductor for enhanced light extractionaccording to the third embodiment of the invention;

FIG. 11 illustrates a conduction and valence band diagram of the LED inFIG. 10 with optical field approximated;

FIG. 12 illustrates a III-nitride photonic crystal assisted resonantcavity LED with a GEMM coupled to a photonic crystal structured p-GaNlayer and nonepitaxial transparent conductor for enhanced lightextraction according to the fourth embodiment of the invention;

FIG. 13 a illustrates a conduction and valence band diagram of the LEDin FIG. 12 with optical field approximated according to the fourthembodiment of the invention;

FIG. 13 b illustrates a photonic crystal structure being utilized toextract guided modes according to the fourth embodiment of theinvention;

FIG. 14 illustrates a process flow diagram for LED of FIG. 6;

FIG. 15 illustrates a process flow diagram for LED of FIG. 8;

FIG. 16 illustrates a Process flow diagram for LED of FIG. 10;

FIG. 17 illustrates a Process flow diagram for LED of FIG. 12.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description of the embodiments, reference is made tothe accompanying drawings which form a part of hereof, and in which isshown by way of illustration specific embodiments in which the inventionmay be practiced. In the drawings, the thickness of layers and regionsare exaggerated for clarity. These embodiments are provided so that thisdisclosure will be thorough and complete, and will convey the teachingsto those skilled in the art. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Semiconductor light emitting structures and devices are described wherethe emitting structure is grown upon a GEMM where the GEMM has beenepitaxially grown above previously grown III-nitride layers or layer,thus allowing the GEMM to be positioned with great precision andaccuracy within the epitaxial structure for the purpose of creatingoptical cavity effects (including microcavity effects) for greater lightextraction. Moreover, due to the mirror being grown epitaxially, thethickness of the mirror may be controlled to form either asemi-reflector, or non-transparent reflector. Various structures of asingle mirrored or duel mirrored RCLED or MCLED may be grown andfabricated by use of one or more GEMM layer as well as other structures.

The GEMM may be positioned with respect to an active region such thatemitted light from the active region and the light reflected from theGEMM constructively interfere to create optical cavity effects thatenhances device efficiency and tailors the spectral purity and spectraldirectionality for different applications in lighting, fiber opticcommunications, biological agent detection, flat panel displays andother applications.

Having the GEMM and the active region in an optimally coupledconfiguration may allow for a second mirror, a photonic crystal orroughened surface to be additionally coupled to enhance or tailor lightemitting characteristics.

The thickness of the GEMM may be configured during the growth process tofine tune the mirror's reflective and transmissive optical properties,from partially transmissive to fully transmissive. Such control duringepitaxial growth provides the flexibility to optimize optical cavityeffects for a multitude of semiconductor light emitting devices.

The device quality GEMM, which will be further described below, may beplaced within the difficult to access n-type III-nitride layer near theactive region of a conventional III-nitride semiconductor light emitterepitaxial stack. Those skilled in the art will understand that instandard III-nitride LEDs the n-type layer is usually grown prior top-type layer(s) during the epitaxial growth process. Generally, the GEMMprovides a mirror embedded within the epitaxial stack to provide opticalcavity effects despite which doped layer is grown first or where theGEMM layer is grown within the epitaxial stack. The GEMM layer may alsoserve as an embedded highly conductive electric carrier transport layeracross the device to improve current injection into either n-type orp-type layers. Additionally, the GEMM may reduce problematic staticdischarge damage.

The material for the GEMM comprises various metals and metal compoundswhich may be grown closely lattice matched to its base III-nitridestructure, avoiding the difficulties of high dislocation densities.Moreover, the GEMM materials are thermal expansion matched to theIII-nitride structure further avoiding difficulties of cracking anddislocation densities.

The GEMM layer may comprise of refractory metals and/or metalliccompounds, such as the Group IVB metals Zr, Hf, (Hf_(x)Zr_(1-x) where xis equal to or between 0 and 1), and the Transitional Metal DiboridesZrB2, HfB2, YB2 and (Hf_(x)Zr_(y)Y_(z)B₂ where x+y+z=1) and theTransitional Metal Nitrides ZrN, HfN, TiN and (Hf_(x)Zr_(y)Y_(z)N wherex+y+z=1).

The bulk conductivity of the GEMM materials are about 30 times moreconductive than the moderately doped n-type III-nitride layers currentlyincorporated into light emitting structures. The highly conductiveproperties of the GEMM may serve to improve the electronic carriertransport across the device, creating pathways for current to flow, andreduce the impact of unwanted static discharge.

The superior conductivity of the GEMM may make it attractive toepitaxially grow the highly resistive p-type materials of theIII-nitride light emitting devices prior to the active region and then-type material allowing for more design flexibility for surfacetexturing of the top n-type epitaxial layer, away from the substrateside.

Technical Considerations Using GEMM

The details of the photon-emission process in LEDs are, in general,related to the coupling of the light emission with electromagneticmodes.

Without the introduction of a cavity, a uniform spontaneous emitteremits evenly within the bulk material and emission covers all 47π ofsolid angle.

Of all the light emitted inside the bulk material, only the emissionthat propagates between the normal and the critical angleθ_(c)=sin⁻¹(N_(OUT)/N_(GaN)) to the interface of the bulk material andthe external medium may escape from the bulk material into the externalmedium, whether or not, there is a cavity. This escape “window” is knownas Snell's window.

The extraction efficiency of the LED is equal to the ratio of modes thatfall within the critical angle θ_(c), compared to the modes that aregenerated.

When the optical electromagnetic modes are confined in one or moredimensions there is a reorganization of what could be considered a freespace optical mode density. This rearrangement of the free space opticalmode density diminishes some frequencies and enhances others while alsochanging the path of the emission.

The cavity extracted emission only propagates within discrete modes, andthus directions governed by the cavity. Each mode equally contains aportion of the total emitted power.

The extraction efficiency of a standard LED or RCLED is equal to theamount of light that escapes the Snell's window compared to the amountof light generated. However, the changes of the RCLEDs internal emissiondistribution due to the optical confinement directs more light out ofSnell's window than the bulk type LED.

Although the RCLED has a higher emission output than a standard LED,changing the cavity length of a RCLED, for the most part, does notchange the emission output as the active region is placed at or close tovarious antinodes of a standing optical wave. In the RCLED regime, asthe cavity length is decreased, the ratio of modes extracted throughSnell's window to guided modes is almost constant as the active regionis placed at or close to various antinodes of a standing optical wave.

However, a transition occurs when the cavity length decreases to a pointwhere the cavity order, Int[2N_(GaN)L_(c)/λ], is smaller than 2N² _(GaN)(note: the function Int[X] rounds X to the nearest integer, N_(GaN) isthe index of refraction for GaN, λ is the emission wave length invacuum, L_(c) is the cavity length). The number of modes extractedthrough Snell's window stays constant (i.e., one mode) whereas thenumber of guided modes continue to decrease, increasing the LEDsextraction efficiency. This regime of cavity length is called themicrocavity regime and the RCLEDs that operate in this regime are calledMCLEDs.

By employing a planar specular GEMM close to the active region of anLED, nano-optical cavity effects take place. The GEMM may be thick to bereflective so that the emission is directed away from the GEMM to beextracted through the top of the device, away from the substrate side.The GEMM may be thin to be semi-reflective so that the emission isdirected through the semi-reflector to be extracted through Snell'swindow. Such a GEMM would be very useful in numerous LED structures asdescribed herein.

To use the GEMM, as described above, it should have certain propertiesto allow for true epitaxial growth and commercialization of the MCLED,RCLED or engineered template. The GEMM should have to be compatible withIII-nitride materials in the properties of lattice constant, thermalexpansion coefficient, temperature stability, reflectivity, electricalproperties, etc.

Being that standing optical waves have a periodic function, a cavitylight emitter may be such that the cavity is of an appropriate lengthand that the active region is placed at or close to an antinode of thestanding optical wave. The structuring of the cavities herein follow therelationships where the cavity length L_(c) is equal to d1+d2 as shownin FIG. 5 where:

$\begin{matrix}{{{d\; 1} = {{the}\mspace{14mu} {distance}\mspace{14mu} {between}\mspace{14mu} {the}\mspace{14mu} {center}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {active}\mspace{14mu} {region}\mspace{14mu} {and}\mspace{14mu} {the}}}\mspace{14mu}} \\{{GEMM}} \\{= {( {0.25{\lambda/N}} ) + {M( {{0{.5}}{\lambda/N}} )} - {{PD}\; 1.}}}\end{matrix}$ $\begin{matrix}{{d\; 2} = {{the}\mspace{14mu} {distance}\mspace{14mu} {between}\mspace{14mu} {the}\mspace{14mu} {center}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {active}\mspace{14mu} {region}\mspace{14mu} {and}}} \\{{a\mspace{14mu} {second}\mspace{14mu} {mirror}\mspace{14mu} {or}\mspace{14mu} {reflective}\mspace{14mu} {interface}}} \\{{= {( {0.25{\lambda/N}} ) + {M( {{0{.5}}{\lambda/N}} )} - {{PD}\; 2\mspace{14mu} {if}\mspace{14mu} {the}\mspace{14mu} {second}\mspace{14mu} {mirror}\mspace{14mu} {is}\mspace{14mu} a\mspace{14mu} {metal}}}};} \\{{and}} \\{= {( {0.75{\lambda/N}} ) + {{M( {{0{.5}}{\lambda/N}} )}\mspace{14mu} {if}\mspace{14mu} {the}\mspace{14mu} {second}\mspace{14mu} {mirror}\mspace{14mu} {is}\mspace{14mu} a\mspace{14mu} {DBR}\mspace{14mu} {or}}}} \\{{{roughened}\mspace{14mu} {{surface}.}}}\end{matrix}$

-   -   λ is the wave length of light generated by the active region in        vacuum.    -   N is the index of refraction of the III-nitride material.    -   M is an integral multiple of half wave lengths within the        III-nitride material starting at zero.    -   PD1 and PD2 are the metal penetration depths of the light into        the GEMM and the second mirror (if metal), respectively.

As the thickness of the GEMM layer is modified the resulting position ofthe PD (penetration depth) needs to be modified.

To be clear, the second mirror or extraction interface is positioned atthe top of the p-GaN layer (508) in FIG. 5 away from the substrate(501).

To ensure that the active region is placed only at or close to anantinode of the standing optical wave, the thickness of the emittingregion may be restricted to a value less than 0.25λ/N. If the activeregion is positioned in such a way that it is not centered and extendsoutside of this width towards a node of the same mode the efficiency maybe degraded.

The error for the placement of such an active region is dependent on thewidth of the active region. The thicker the active region is the lesserror there is for optimal performance. The error may be considered:0.125(λ/N)-0.5(W_(AR)), where W_(AR) is the active region width, asillustrated in FIG. 5. If the error involved with the placement of theactive region is breached the device will work however inefficientlyunless other light extracting structures are made available such as aphotonic crystal or crystals and roughed surfaces or other deviceshaping deviating from the standard LED.

Furthermore, a roughened surface or photonic crystal may assist in thelight extraction of guided modes or mode. Moreover, the device may bedesigned that the guided elements are intentionally coupled to thefeatures of the roughened or photonic crystal structures to form a“guided extraction mode.” The device structure may be optimized to usethe GEMM to exclusively extract guided modes through the use ofmicro-optical-cavity effects, specifically through the rearrangement ofthe freespace optical mode density. These methods may be in one or moreembodiments below.

First Embodiment

The structure and band diagram of the first embodiment are shown in FIG.6 and FIG. 7, respectively.

The GEMM (604) lends itself to many forms of optical cavitysemiconductor light emitters. In the first embodiment the GEMM (604),with its highly reflective bulk properties, may be grown thin such thatthe GEMM (604) is partially transparent and partially reflective. Thefirst embodiment utilizes a planar III-nitride material growth templatecomprising of sapphire substrate (601), i-GaN buffer layer (602) andn-GaN layer (603) of device quality so that the crystal quality of theGEMM (604) is planar, specular and of device quality. The GEMM (604) islattice matched to the n-GaN layer (603) such that the desired thicknessmay be below the epitaxial critical thickness.

Once the 2 μm i-GaN buffer (602) and the 2 μm n-GaN (603) layers aregrown on a sapphire substrate (601) the GEMM (604) may be grown to aspecific thickness (˜20 nm) for the resonant cavity (611) such that theGEMM is partially reflective and partially transparent. Once the GEMM(604) is grown on the n-GaN layer (603) the second n-GaN layer (605) maybe grown to a specific thickness (˜1200 Å) that is optimal for theresonant cavity (611) being that the active region (606) may bepositioned at or close to an antinode (702) being a maximum the of theoptical field (701) within a width of 0.25×/N_(GaN) as shown in FIG. 7(N_(GaN) is the index of refraction, λ is the emission wave length invacuum).

In the first embodiment, the illustrated RCLED may emit at a wavelengthof 500 nm with a cavity length (611) of about 3000 Å (being within themicrocavity regime). The light generated in the active region (606)resonates between the thick, highly reflective non-epitaxial metalmirror (610) and the thin, partially reflective, partially transparent,GEMM (604). The cavity is such that the mirrors (604) (610) are spacedapart such that light generated by the active region (606) may resonatebetween the mirrors. The mirrors (604) (610) are spaced apart such thatan integer number of half wave lengths of the light generated by theactive region (606) may fit between the mirrors (604) (610) within thenitride materials. The active region (606) is placed in between themirrors such that the generated light constructively interferes with thereflected light from both mirrors (604) (610). The resonating lightexits the GEMM (604) side of the device through the sapphire substrate(601), as shown in FIGS. 6 and 7.

Positioning the center of the active region (606) of the MCLED at, orclose to, a maximum (702) of the optical field distribution (701) isshown in FIG. 7. The local maximums (702) of the optical fielddistribution (701) are referred to as antinodes (702) because they areantinodes of the standing optical wave. Positioning the center of theactive region (606) at or close to the closest antinodes to thenon-epitaxial metal mirror (610) contact and the GEMM (604) is desiredto reduce the number of guided modes and increase the light extractionthrough Snell's window. As shown in FIG. 7, antinodes (702) of thestanding optical wave are positioned periodically away from thenonepitaxial metal mirror contact (610) and the GEMM (604).

The following factors should be considered for this embodiment:

-   -   a. the n-type GaN layer template (603) being of device quality        and planar,    -   b. the thickness, flatness, specular reflectivity and        transparency of the GEMM layer (604) grown on the first n-type        GaN layer (603),    -   c. the thickness of second n-type conductive layer (605) grown        on the GEMM (604) and its being of device quality,    -   d. the position of the active region (606) relative to the        enclosing mirrors (604), (610),    -   e. the thickness of the p-type conductive layers (607) (608),        and    -   f. the reflective quality of the non-epitaxial metal contact        (610).

Although the device efficiency is superior to that of a standard LED, itmay be of interest to detune the device in its construction such thatthe enhancements in directionality and spectral narrowing aresubstituted for even higher light extraction efficiency. This may beperformed by keeping all parameters the same while increasing thewavelength of the device by adjusting the quantum well composition orwidth. This forces the standing optical wave into a tilted position awayfrom the normal of the device. This adjustment may only be done suchthat the angle is within Snell's window; otherwise the light outside ofthe critical angel is reflected back into the device. The critical angleof Snell's window is defined as sin⁻¹(N_(exit)/N_(GaN)) where N_(exit)is the index of refraction external of the device and N_(GaN) is theindex of refraction of Gallium Nitride.

Although laser lift off is not required to form the MCLED, there may bebenefits to removing the sapphire substrate (601) for thermal extractionor n-side surface roughening. Moreover, if the substrate isnon-transparent (i.e., silicon) the silicon substrate may be selectivelyetched away, forming the functional device structure of FIG. 6, withoutthe substrate, whether or not there is a buffer between the GEMM or not.

This example is not meant to be restrictive in that the cavity length,emission wavelength and structural aspects may be changed such that theactive region is positioned optimally within the optical field of theextraction mode for a given emission wavelength and direction.

One example of a modification is utilizing a thinner non-epitaxial metalmirror (610) such that the thickness is about 20 nm like the GEMM (604)layer to emit light from both sides of the planar device.

Another example of a modification to the structure outlined in FIG. 6 isto position a 20 μm GEMM layer on or within the p-GaN layer and replacethe thick non-epitaxial metal mirror with a transparent metal contactsuch that light may escape from the top and the bottom of the device.

Another example comprises of a photonic crystal embedded within thestructure of FIG. 6 to extract guided modes or to prevent guided modesfrom forming. This will be discussed more in the fourth embodiment.

Second Embodiment

The structure and band diagram of the second embodiment are shown inFIG. 8 and FIG. 9, respectively.

The GEMM (804) lends itself to many forms of optical cavitysemiconductor light emitters. In the second embodiment the GEMM (804),with its highly reflective bulk properties, may be grown thick such thatthe GEMM (804) takes on its bulk reflective properties. The secondembodiment utilizes a planar III-nitride material growth templatecomprising of sapphire substrate (801), i-GaN buffer layer (802) andn-GaN layer (803) of device quality so that the crystal quality of theGEMM (804) is planar, specular and of device quality. The GEMM (804) islattice matched to the n-GaN layer (803) such that the desired thicknessmay be below the epitaxial critical thickness.

Once the 2 μm i-GaN buffer (802) and the 2 μm n-GaN (803) layers aregrown on a sapphire substrate (801) the GEMM (804) may be grown to athickness (≧(greater than approximately) 150 nm) for the resonant cavity(812). Once the GEMM (804) is grown the second n-GaN layer (805) may begrown to a specific thickness (˜1260 Å) that is optimal for the resonantcavity (812) being that the active region (806) may be positioned at thelocal maximums (902) of the optical field (901) within a width of0.25λ/N_(GaN) as shown in FIG. 9 (N_(GaN) is the index of refraction, λis the emission wave length in vacuum).

In the second embodiment, the illustrated MCLED may emit at 500 nm witha cavity length (812) of about 2600 Å (being within the microcavityregime). The light being generated in the active region (806) resonatesbetween the thick GEMM (804) and the interface (813) of the p-GaN layer(808) and the transparent contact (810). The cavity is such that themirrors (804) (813) are spaced apart such that light generated by theactive region (806) may resonate between the mirrors. The mirrors (804)(813) are spaced apart such that an integer number of half wave lengthsplus one quarter wave length of the light generated by the active region(806) may fit between the mirrors (804) (813) within the nitridematerials. The active region (806) is placed in between the mirrors suchthat the generated light constructively interferes with the reflectedlight. The resonating light exits the transparent contact (810) side ofthe device.

The positioning of the center of the active region (806) of the LED at,or close to, a maximum (902) of the optical field distribution (901) isshown in FIG. 9. The local maximums (902) of the optical fielddistribution (901) are referred to as antinodes (902) because they arethe antinodes of the standing optical wave. Positioning the center ofthe active region (806) at or close to the closest antinodes to themirrors is desired to reduce the number of guided modes and increase thelight extraction through Snell's window. Positioning the mirrors so thatthe cavity length is small is most favored. As shown in FIG. 9,antinodes (902), of the standing optical wave, are positionedperiodically away from the p-GaN/transparent conducting interface (813)and the GEMM (804).

The following factors should be considered for this embodiment:

-   -   a. the n-type GaN layer (803) or i-GaN buffer layer (802)        template being of device quality and planar,    -   b. the thickness, flatness, and specular reflectivity of the        GEMM layer (804) grown on the first n-type GaN layer (803),    -   c. the thickness of second n-type conductive layer (805) grown        on the GEMM (804) and its being of device quality,    -   d. the position of the active region (806) relative to the        enclosing mirrors (804), (813),    -   e. the thickness of the p-type conductive layers (807) (808),        and    -   f. the reflective quality of p-GaN/transparent conductor        interface (813) and the thickness of the transparent conductor        (810).

Although the efficiency of this device is superior to that of a standardLED, it may be of interest to detune the device in its construction suchthat the enhancements in directionality and spectral narrowing aresubstituted for even higher light extraction efficiency. This may beperformed by keeping all parameters the same while increasing thewavelength of the device by adjusting the quantum well composition orwidth. This forces the standing optical wave into a tilted position awayfrom the normal of the device. This adjustment may only be done suchthat the angle is within Snell's window; otherwise the light outside ofthe critical angel is reflected back into the device. The critical angleof Snell's window is defined as sin⁻¹(N_(exit)/N_(GaN)) where N_(exit)is the index of refraction outside of the device and N_(GaN) is theindex of refraction of Gallium Nitride.

Although laser lift off is not required to form the MCLED there arebenefits to removing the sapphire substrate (801) for thermal extractionand current injection from the removed substrate side of the device.

This example is not meant to be restrictive in that the cavity length,emission wavelength and structural aspects may be changed such that theactive region is positioned optimally within the optical field of thecavity for a given emission wavelength and direction.

One example of a modification to the structure outlined in FIG. 8 is a˜20 nm GEMM positioned within or on the p-GaN layer (808) such that aGEMM layer is located at or close to a node in addition to the alreadypresent thick GEMM layer (804). The emitted light exits the p-GaN layer(808) side of the device. Additionally, in this way, a MCLED having acavity order of one, may be achieved allowing for a ˜90% lightextraction efficiency if the mirrors enclose one antinode.

Third Embodiment

The structure and band diagram of the third embodiment are shown in FIG.10 and FIG. 11, respectively.

The GEMM (1004) lends itself to many forms of optical cavitysemiconductor light emitters. In the third embodiment the GEMM (1004),with its highly reflective bulk properties, may be grown thick such thatthe GEMM (1004) takes on its bulk reflective properties. The thirdembodiment utilizes a planar III-nitride material growth templatecomprising of sapphire substrate (1001), i-GaN buffer layer (1002) andn-GaN layer (1003) of device quality so that the crystal quality of theGEMM (1004) is planar, specular and of device quality. The GEMM (1004)is lattice matched to the III-nitride n-GaN layer (1003) such that thedesired thickness may be below the epitaxial critical thickness.

Once the 2 μm i-GaN buffer (1002) and the 2 μm n-GaN (1003) layers aregrown on a sapphire substrate (1001) the GEMM (1004) may be grown to athickness (≧(greater than approximately) 150 nm) for the surfaceroughened assisted resonant cavity (1012). Once the GEMM (1004) is grownon the template the second n-GaN layer (1005) may be grown to a specificthickness (˜1260 Å) that is optimal for the optical cavity (1012)effects being that the center of the active region (1006) may bepositioned at a local maximum (1102) of the optical field (1101) withina width of 0.25λ/N_(GaN) as shown in FIG. 11 (N_(GaN) is the index ofrefraction, λ is the emission wave length in vacuum).

The third embodiment with roughened p-GaN layer (1008) has severalvariations being that the roughened surface has features close to, orgreater than, the wavelength of the emitted light within thesemiconductor. The features may be random or ordered.

The roughened surfaces are not drawn to scale in FIGS. 10 and 11. Thefeatures are meant to only be representative.

If the features are random, the device may operate in such a way thatthe photons are given multiple opportunities to find an escape cone. Ifthe light does not pass through Snell's window initially, the randomreflecting surfaces, of the roughened interface, redirect emission atvarious angles towards the GEMM layer to be reflected to anotherposition along the roughened interface. This process continues until thelight is absorbed or escapes the light emitting device.

If the features are ordered the device may still operate in such a waythat the photons are given multiple opportunities to find an escape coneif reflected back, however much of the light emitted from the activeregion toward the roughened p-type GaN layer surface may be scattered ordiffracted and extracted.

Although the surface is roughened, Snell's law still applies;consideration must be made when determining the surface features suchthat the most of the impinging emission escapes. If the lightpropagating normal to the device is to escape the device, the anglebetween the normal of the surface feature and the normal of the deviceshould be below the critical angle θ_(c)=sin(N_(OUT)/N_(GaN)).

The illustrated optical cavity LED may emit at 500 nm with an effectivecavity length (1012) of about 2600 Å (being within the microcavityregime). The light being generated in the active region (1006) selfinterferes constructively with the light reflecting from the thick GEMM(1004). Generally, the roughened surface should be positioned from theactive region such that the semiconductor interface (1013) is positionedat or close to an antinode. The constructively interfering light exitsthe transparent contact (1010) side of the device.

Positioning the center of the active region (1006) of the LED at, orclose to, a maximum (1102) of the optical field distribution (1101) isshown in FIG. 11. The local maximums (1102) of the optical fielddistribution (1101) are referred to as antinodes (1102) because they areantinodes of the standing optical wave. Positioning the center of theactive region (1006) at or close to the closest antinodes to the GEMM(1004) is desired to reduce the number of guided modes and increase thelight extraction through Snell's window. As shown in FIG. 11, antinodes(1102), of the standing optical wave, are positioned periodically awayfrom the GEMM (1004).

The following factors should be considered for this embodiment:

-   -   a. the n-type GaN layer (1003) or i-GaN buffer layer (1002)        template being of device quality and planar,    -   b. the thickness, flatness, and specular reflectivity of the        GEMM layer (1004) grown on the first n-type GaN layer (1003),    -   c. the thickness of second n-type conductive layer (1005) grown        on the GEMM (1004) and its being of device quality,    -   d. the position of the active region (1006) relative to the        surrounding mirror (1004) and roughened surface,    -   e. the thickness of the p-type layers (1007) (1008), and    -   f. the thickness of the transparent conductor (1010).

Although laser lift off is not required to form the roughened surfaceassisted optical cavity LED there are benefits to removing the sapphiresubstrate for thermal extraction and current injection from the removedsubstrate side of the device.

This example is not meant to be restrictive in that the constructiveinterference length, emission wavelength and structural aspects may bechanged such that the active region is positioned optimally within theoptical field of the constructive interference length for a givenemission wavelength and direction.

One example may be optimized by the emission impinging on the roughenedsurface entering into a surface feature such that the light is reflectedwithin the feature from surface to surface until the light is at anangle such that the light may escape through Snell's window.

Another example may be optimized by exclusively extracting otherwiseguided modes through the use of micro-optical-cavity effects,specifically through the rearrangement of the free space optical modedensity such that the extraction mode is directed perpendicular to theextraction surface which is off axis from the normal of the planarsubstrate and subsequent layers. In this arrangement, verticalextraction modes may not be required and only guided modes or a singleguided mode would be optimal for the device.

Fourth Embodiment

The structure and band diagram of the fourth embodiment are shown inFIG. 12 and FIG. 13 a, respectively.

The GEMM (1204) lends itself to many forms of optical cavitysemiconductor light emitters. In the fourth embodiment the GEMM (1204),with its highly reflective bulk properties may be grown thick such thatthe GEMM (1204) takes on its bulk reflective properties. The fourthembodiment utilizes a planar III-nitride material growth templatecomprising of a sapphire substrate (1201), i-GaN buffer layer (1202) andn-GaN layer (1203) of device quality so that the crystal quality of theGEMM (1204) is planar, specular and of device quality. The GEMM (1204)is lattice matched to the n-GaN layer (1203) such that the desiredthickness may be below the epitaxial critical thickness.

Once the 2 μm i-GaN buffer (1202) and the 2 μm n-GaN (1203) layers aregrown on a sapphire substrate (1201) the GEMM (1204) may be grown to athickness (≧(greater than approximately) 150 nm or greater) that isoptimal for the photonic assisted resonant cavity. It is preferred tokeep this layer below the critical thickness. Once the GEMM (1204) isgrown on the template, the second n-GaN layer (1205) may be grown to aspecific thickness (˜1260 Å) that is optimal for optical cavity effectsbeing that the active region (1206) may be positioned within the localmaximum (1302) of the optical field (1301) within a width of0.25λ/N_(GaN) as shown in FIG. 13 a (N_(GaN) is the index of refraction,λ is the emission wavelength in vacuum).

The photonic crystal assisted resonant cavity LED with wave length scaletwo dimensional periodic structures (1213) may enhance the devicesperformance by adjusting several components: the positioning of the GEMM(1204), grating depth, grating spacing, grating hole width and gratingfill factor. These components may be configured to manipulate guidedmodes (1303) by inhibiting the modes from formation, by extracting theguided modes via diffraction or by concentrating the guided modes viareflection to increase photon recycling or various combinations of thesethree configurations.

In this fourth embodiment an example where the photonic crystal (1214)is used to extract guided modes (1303), as shown in FIG. 13 b, isprovided.

The photonic crystal structures are not drawn to scale in FIG. 12, FIG.13 a and FIG. 13 b. The features are meant to only be representative.

In the fourth embodiment, a photonic crystal (1214) assisted RCLED mayemit at 500 nm with a cavity length (1213) of about 2600 Å (being withinthe microcavity regime). The light being generated in the active region(1206) resonates between the thick GEMM (1204) and the interface (1215)of the p-GaN layer (1208) and the transparent contact (1210). The cavityis such that the mirrors (1204) (1215) are spaced apart such that lightgenerated by the active region (1206) may resonate between the mirrors.The mirrors (1204) (1215) are spaced apart such that an integer numberof half wave lengths plus one quarter wave length of the light generatedby the active region (1206) may fit between the mirrors (1204) (1215)within the nitride materials. The active region (1206) is placed inbetween the mirrors such that the generated light constructivelyinterferes with the reflected light. The resonating light exits thetransparent contact (1210) side of the device.

The guided modes are subject to being diffracted vertically from theemitting surface as shown in FIG. 13 b.

The following factors should be considered for this embodiment:

-   -   a. the first n-type GaN layer (1203) or i-GaN buffer (1202)        template being of device quality and planar,    -   b. the thickness, flatness, and specular reflectivity of the        GEMM layer (1204) grown on the first n-type GaN layer (1203),    -   c. the thickness of second n-type conductive layer (1205) grown        on the GEMM (1204) being of device quality,    -   d. the position of the active region (1206) to the surrounding        mirror (1204), semiconductor interface (1215) and photonic        crystal (1214),    -   e. the thickness of the p-type conductive layers (1207) (1208),        and    -   f. the depth and configuration of the holes (1212) in the        photonic crystal (1214).

Although laser lift off is not required to form the Photonic Crystalassisted RCLED there are benefits to removing the sapphire substrate forthermal extraction and current injection from the removed substrate sideof the device.

This example is not meant to be restrictive being the cavity length,photonic crystal grid and emission wavelength may be of variousconfigurations such that the active region is centered at or close to anantinode within an extraction mode of the cavity and coupled photoniccrystal.

One example of a modification to the structure outlined in FIG. 12comprises of the transparent contact (1210) being replaced with a thick(≧(greater than approximately) 150 nm) reflective metal mirror with areduction of the GEMM layer (1204) to about 20 nm in thickness. Moreoverthe cavity thickness (1213) is adjusted to place both metal mirrorsclose to nodes and the active region remains at or close to an antinode.Additionally, the photonic crystal is designed such that the generatedguided light is diffracted into a vertical mode(s) or that the guidedmodes are inhibited. This configuration allows for the MCLED with aphotonic crystal on the device side of the thick reflective metalmirror.

Although the holes in the p-GaN layer that make up the photonic crystalare not distributed evenly through out the layer of the drawing, thisdoes not indicate that the photonic crystal structure can not be mademore uniform. As indicated, there are several photonic crystalconfigurations.

Fifth, Sixth, Seventh and Eighth Embodiments

The fifth, sixth, seventh and eighth embodiments are similar to thefirst, second, third and fourth embodiments, respectively. The fifth,sixth, seventh and eighth embodiments have a different order in theepitaxial structure from the previous embodiments. In general, then-type layer is grown before the p-type layers as described in theprevious embodiments. The doped epitaxial layers and active region ofthe fifth, sixth, seventh and eighth embodiments grown in reverse orderand summarized in Table I.

To be clear the general epitaxial order of the fifth, sixth, seventh,and eighth embodiments follows: Sapphire\i-GaN\GEMM\p-GaN\p-AlGaN\Activeregion\n-GaN. Examples of the epitaxial structure are summarized inTable I.

TABLE I Layer Thickness doping n-GaN Embodiment 5: ~120 nm 5 ×10{circumflex over ( )}18 cc/cm3 Embodiments 6, 7, 8*: ~126 nm*Embodiment 8: Depends on photonic crystal arrangement Active Region 4X~3 nm InGaN/~6 nm GaN NA p-AlGaN ~19 nm 3 × 10{circumflex over ( )}17cc/cm3 p-GaN ~237 nm (Embodiment 5) 5 × 10{circumflex over ( )}17 cc/cm3~214 nm (Embodiment 6, 7, 8) GEMM ~20 nm (Embodiment 5) NA ≧(greaterthan approximately) 150 nm (Embodiment 6, 7, 8) i-GaN 2 μm NA Sapphiresubstrate 300 μm NA

In the fifth embodiment the processing operations are the same as thefirst, however, a reflective ohmic contact is deposited on the n-GaNlayer.

In the sixth, seventh and eighth embodiments the processing operations(FIG. 15-FIG. 17) are the same as the second, third and fourthembodiments, however, now the n-terminal and the p-terminal aresubstituted in place of the other.

The key aspects of positioning the center of the active region at orclose to an antinode of the designed optical mode for a given emission,as described in the previous embodiments, is applied in these summarizedembodiments.

Epitaxy and Processing

General Epitaxy Growth of the First, Second, Third and FourthEmbodiments

As an example, FIG. 5 illustrates the sequence of the epitaxial layersinvolved within the general base epitaxial structure for the first,second, third and fourth embodiments. Material layers are grown usingOrgano-Metalic Vapor Phase Epitaxy (OMVPE), Molecular Beam Epitaxy(MBE), Hydride Vapor Phase Epitaxy (HYPE), Physical Vapor Deposition(PVD) and the like, whether being a single tool or combinations and/orvariations thereof. The growth conditions to produce these layers aresubject to the tools used. For some embodiments an epitaxial structurecomprises:

1. The growth of epitaxial material on a sapphire substrate (501). Thequality of the subsequent epitaxial layers (502, 503, 504, 505, 506,507, 508) that are formed above this substrate (501) may be dependant onthe substrate (501) orientation, surface roughness and/or surfacetreatment.

2. Intrinsically doped GaN buffer layer (502). The layer isintrinsically doped GaN buffer layer that is about 2 μm thick. Thequality of the subsequent epitaxial layers (503, 504, 505, 506, 507,508) that are formed above this layer (502) may be dependent on thethickness of this layer and initial growth conditions when formed on thesubstrate (501).

3. Doped n-type GaN layer (503). The layer is 2 μm of Si-doped GaN whichacts as an n-type layer for electrical conduction with the GEMM layer(504) within the processed light emitter. The density of charge carriersis about 5 times 10̂18 per cubic centimeter. The doping of this layernear the above GEMM (504) interface may be higher or lower to optimizeohmic operation. The quality of the subsequent epitaxial layers that areformed above this layer may be dependent on the thickness of this layer.

4. GEMM layer (504). The crystalline quality of this layer is dependenton the growth conditions and the crystalline quality of the previouslayers (502) (503) and Substrate (501).

The GEMM layer (504) comprises of refractory metals and/or metalliccompounds, such as the Group IVB metals (Zr, Hf and the like(Hf_(x)Zr_(1-x))), the Transitional Metal Diborides (ZrB2, HfB2, YB2 andthe like (Hf_(x)Zr_(y)Y_(z)B₂ where x+y+z=1)) and Transitional MetalNitrides (ZrN, HfN, TiN and the like (Hf_(x)Zr_(y)Y_(z)N wherex+y+z=1)).

As further examples, the thin GEMM (504) (604) used in the first andfifth embodiments may be about 20 nm in thickness such that it ispartially transparent, reflective and weakly absorbing.

The thick GEMM (504) (804) (1004) (1204) used in the second, third,fourth and sixth, seventh eighth embodiments may be greater than about(greater than approximately) 150 nm in thickness such that it may takeon its bulk reflective properties.

The thickness of the GEMM may be epitaxially grown below the crystallinerelaxation critical thickness.

In the second (see FIG. 8 and FIG. 9) and sixth, third (see FIG. 10 andFIG. 11) and seventh, and fourth (see FIG. 12 and FIG. 13) and eighthembodiments, the devices may be designed such that the generated lightis to propagate through the device, away from the sapphire substrate,such that the thickness of the GEMM (504), (804), (1004), (1204) isthick enough to reflect with no light transmitting though the GEMM(504), (804), (1004), (1204).

The composition of the GEMM (504) may be such that this layer (504) maybe lattice matched to the layers above and below. The GEMM (504) may bebelow the critical thickness. Critical thickness is a thickness wherethe built up strain due to lattice mismatch, composition and thicknessis relaxed by forming defects in the material.

5. Thin Doped n-type layer (505). Si-doped GaN may act as an n-typelayer in the light emitter and is one side of an optical cavity furthercomprising a single mirror or dual mirrors. The layer thickness may besuch that the active region (506) should be located at or close to anantinode of the standing optical wave (maximum of the optical field).The density of charge carriers may be about 5 times 10̂18 per cubiccentimeter. The doping of this layer near the interface of the belowlayer may be higher or lower to optimize the ohmic operation.

6. Active region of the light emitter (506). The active region may becomprised of 4 InGaN quantum wells 3 nm thick separated by undoped GaNbarrier layers 6 nm thick. The quantum wells indium composition may bechosen to emit at any wavelength within the nitride wavelength range.Various thicknesses and compositions of quantum wells and thicknesses ofthe barriers may be grown, as long as the active region is localized ator close to an antinode of the standing optical wave. The active region(506) may be less than λ/(4N) thick to position the active region at orclose to an antinode of the cavity where λ is the wave length of lightgenerated by the active region in vacuum, N is the index of refractionof the III-nitride material.

7. Doped p-type AlGaN layer (507). This magnesium doped AlGaN layer(507) may be about 19 nm thick and may be an electron blocking layer.This layer may be thicker or thinner or may be omitted.

8. Doped p-type GaN layer (508). This magnesium doped GaN layer (508)may act as the p-type layer for electrical contact (610), (810), (1010),(1210) in the light emitting emitter. In the third embodiment (see FIG.10 and FIG. 11), this layer (508), (1008) may be grown with such growthconditions so that the surface roughness is purposely enhanced.

General Epitaxy Growth of the Fifth, Sixth, Seventh and EighthEmbodiments

The epitaxial process for the fifth, sixth, seventh and eighthembodiments are similar to the previous embodiments. The structures arepresented in Table I.

In these embodiments it may be desired to reduce possible Mgcontamination within the active region by growing the p-GaN and p-AlGaNlayers in a separate chamber from the rest of the semiconductor growth.

Processing Steps of First Embodiment

FIG. 14 is a flow chart illustrating exemplary processing steps tofabricate a MCLED or RCLED that may result from the epitaxial structureof FIG. 6. The device may be operated so that the light exits thetransparent substrate (501) (601). The flow chart starts with theepitaxial structure of FIG. 5 as the base structure.

Block 1401 represents the step of activation of the p-GaN layer (508)(608) and/or p-AlGaN layer (507) (607). This comprises of activation ofthe Mg-doped layers thermally to reduce the p-GaN layer (508) (608)and/or p-AlGaN layer (507) (607) resistivity. The sample is heated to710° C. for 10 minutes in an atmosphere of O₂ and N₂.

Block 1402 represents the steps of mesa formation and n-GaN layer (503)(603)/thin GEMM (504) (604) contact etch. The mesas may be patterned byphotolithography and etched by reactive ion etching to isolate theindividual devices. Contacts may be patterned by photolithography on themesas and etched by reactive ion etching to expose the n-GaN layer (503)(603) or epitaxial metal (504) (604) so that an electrical terminal(609) may be deposited later. The sample is etched in a chlorineatmosphere. Other etching techniques may be used.

Block 1403 represents the step of p-contact terminal deposition on thetop of the mesa. A silver mirror (610) may be deposited by electron beamdeposition to act as the electrical contact to the p-type GaN layer(508) (608). Any material that makes a good contact with the p-doped GaNlayer and acts as a highly reflective metal mirror may be used. Othermetal deposition techniques may be used.

Block 1404 represents the step of n-contact terminal deposition on topof the n-GaN layer and/or thin GEMM. The electrical contact of then-doped GaN layer (503) (603) and/or thin GEMM (504) (604) may bedefined by photolithography and then deposited by electron beamdeposition on the n-GaN layer (503) (603) and/or thin GEMM (504) (604).The contact is typically comprised of Ti/Al/Ni/Au (609). A ring shapedcontact may be used to maximize the electrical injection into thedevice. Other metals and metal deposition techniques may be used.

Block 1405 represents the step of chemical mechanical polish. The backside of the transparent substrate (501) (601) may be thinned at thispoint for better thermal extraction and ease of die separation.

Block 1406 represents the step of die separation. The die separation maybe performed by a scribe-and-break process, by a laser separation or asawing process.

Processing Steps of Second Embodiment

FIG. 15 is a flow chart illustrating exemplary processing steps tofabricate a MCLED, or RCLED that may result from the epitaxial structureof FIG. 8. The device may be operated so that the light exits thetransparent contact (810). The flow chart starts with the epitaxialstructure of FIG. 5 as the base structure.

Block 1501 represents the step of activation of the p-GaN layer (508)(808) and/or p-AlGaN layer (507) (807). This comprises of activation ofthe Mg-doped layers thermally to reduce p-type GaN layer (508) (808) andp-type AlGaN layer (507) (807) resistivity. The sample is heated to 710°C. for 10 minutes in an atmosphere of O₂ and N₂.

Block 1502 represents the steps of mesa formation and n-GaN layer (503)(803)/GEMM contact (504) (804) etch. The mesas may be patterned byphotolithography and etched by reactive ion etching to isolate theindividual devices. Contacts may be patterned by photolithography on themesas and etched by reactive ion etching to expose the n-GaN layer (503)(803) or the GEMM layer (504) (804) so that an electrical terminal canbe deposited later. The sample may be etched in a chlorine atmosphere.Other etching techniques may be used.

Block 1503 represents the step of p-contact deposition on the top of themesa. A Ni/Au, ITO or any other transparent ohmic contact may bedeposited by electron beam deposition to act as the electrical contactto the p-type GaN layer (508) (808). The top interface of the p-GaNlayer (508) (808) layer performs as one side of the cavity. Other metalsand metal deposition techniques may be used.

Block 1504 represents the step of p-contact terminal deposition on thetop of the transparent ohmic p-contact (810). A p-type electrode (811)may be then formed on one side of the transparent conductive layer(810). The p-type electrode (811) may be made of any suitable materialincluding, for example, Ni/Au, Pd/Au, Pd/Ni and Pt.

Block 1505 represents the step of n-contact terminal deposition on topof the n-GaN layer (503) (803) or (505) (805) and/or GEMM (504) (804).The electrical contact may be defined by photolithography and thendeposited by electron beam deposition. The contact is typicallycomprised of Ti/Al/Ni/Au. A ring shaped contact may be used to maximizethe electrical injection into the device. Other metals and metaldeposition techniques may be used.

Block 1506 represents the step of chemical mechanical polish. The backside of the transparent substrate may be thinned at this point forbetter thermal extraction and ease of die separation.

Block 1507 represents the step of die separation. The die separation maybe performed by a scribe-and-break process, by a laser separation or asawing process.

Processing Steps of Third Embodiment

FIG. 16 is a flow chart illustrating exemplary processing steps tofabricate a Roughened Surface Assisted MCLED, or RCLED that may resultfrom the epitaxial structure of FIG. 10. The device may be operated sothat the light exits the transparent contact (1010). The flow chartstarts with the epitaxial structure of FIG. 5 as the base structure.

Block 1601 represents the roughening of the p-GaN layer (508) (1008).The roughening of the p-GaN layer (508) (1008) may be performed usingseveral techniques. Some techniques include:

-   -   a. cooler temperatures of the growth window during p-GaN layer        (508) (1008) growth,    -   b. electrochemical etching with or without photolithography,    -   c. chemical etching with or without photolithography,    -   d. ion etching with or without photolithography.

Block 1602 represents the step of activation of the p-GaN layer (508)(1008) and p-AlGaN layer (507) (1007). This comprises of activation ofthe Mg-doped layers thermally to reduce p-type GaN layer (508) (1008)and p-type AlGaN layer (507) (1007) resistivity. The sample is heated to710° C. for 10 minutes in an atmosphere of O₂ and N₂.

Block 1603 represents the steps of mesa formation and n-GaN layer (503)(1003)/thick-GEMM contact (504) (1004) etch. The mesas may be patternedby photolithography and etched by reactive ion etching to isolate theindividual devices. Contacts may be patterned by photolithography on themesas and etched by reactive ion etching to expose the n-GaN layer (503)(1003) and/or GEMM (504) (1004) so that an electrical terminal may bedeposited later. The sample may be etched in a chlorine atmosphere.Other etching techniques may be used.

Block 1604 represents the step of p-contact deposition on the top of themesa. A Ni/Au, ITO or any other transparent ohmic contact may bedeposited by electron beam deposition to act as the electrical contactto the p-type GaN layer (508) (1008). The bottom interface with thep-GaN layer (508) (1008) performs as one side of the cavity. Othermetals and metal deposition techniques may be used.

Block 1605 represents the step of p-contact terminal deposition on thetop of the transparent ohmic p-contact (1010). A p-type electrode (1011)may be then formed on one side of the transparent conductive layer(1010). The p-type electrode (1011) may be made of any suitable materialincluding, for example, Ni/Au, Pd/Au, Pd/Ni and Pt.

Block 1606 represents the step of n-contact terminal deposition on topof the lower n-GaN layer (503) (1003) and/or thick GEMM (504) (1004)and/or upper n-GaN layer (505) (1005). The electrical contact (1009) maybe defined by photolithography and then deposited by electron beamdeposition. The contact (1009) is typically comprised of Ti/Al/Ni/Au. Aring shaped contact (1009) may be used to maximize the electricalinjection into the device. Other metals and metal deposition techniquesmay be used.

Block 1607 represents the step of chemical mechanical polish. The backside of the transparent substrate may be thinned at this point forbetter thermal extraction and ease of die separation.

Block 1608 represents the step of die separation. The die separation maybe performed by a scribe-and-break process, by a laser separation or asawing process.

Processing of Fourth Embodiment

FIG. 17 is a flow chart illustrating exemplary processing steps tofabricate a Photonic crystal (1214) Assisted MCLED, or RCLED that mayresult from the epitaxial structure of FIG. 12. The device may beoperated so that the light exits the transparent-contact(1210)/photonic-crystal (1214) as illustrated in FIG. 12. The flow chartstarts with the epitaxial structure of FIG. 5 as the base structure.

Block 1701 represents the step of activation of the p-GaN layer (508)(1208) and p-AlGaN layer (507) (1207). This comprises of activation ofthe Mg-doped layers thermally to reduce p-type GaN layer (508) (1208)and p-type AlGaN layer (507) (1207) resistivity. The sample is heated to710° C. for 10 minutes in an atmosphere of O₂ and N₂.

Block 1702 represents the step of p-contact deposition on the top of themesa. A Ni/Au, ITO or any other transparent ohmic contact may bedeposited by electron beam deposition to act as the electrical contactto the p-type GaN layer. Other metals and metal deposition techniquesmay be used.

Block 1703 represents the steps of mesa formation and n-GaN layer (503)(1203)/GEMM contact (504) (1204) etch. The mesas may be patterned byphotolithography and etched by reactive ion etching to isolate theindividual devices. Contacts may be patterned by photolithography on themesas and etched by reactive ion etching to expose the n-GaN layer (503)(1203) or thick GEMM (504) (1204) so that an electrical terminal may bedeposited later. The sample may be etched in a chlorine atmosphere.Other etching techniques may be used.

Block 1704 represents the etching of holes (1212) into the p-GaN layer(1208). The etching of the p-type layers (1207) (1208) may be performedusing several techniques. Some techniques include:

-   -   a. electrochemical etching with photolithography,    -   b. chemical etching with photolithography,    -   c. ion Etching with photolithography,    -   d. focused ion beam.

Block 1705 represents the step of p-contact terminal deposition on thetop of the transparent ohmic p-contact (1210). A p-type electrode (1211)may then be formed on one side of the transparent conductive layer(1210). The p-type electrode (1211) may be made of any suitable materialincluding, for example, Ni/Au, Pd/Au, Pd/Ni and Pt.

Block 1706 represents the step of n-contact terminal deposition on topof the n-GaN layer (503) (1203) or (505) (1205) and/or thick GEMM (504)(1204). The electrical contact may be defined by photolithography andthen deposited by electron beam deposition. The contact is typicallycomprised of Ti/Al/Ni/Au. A ring shaped contact may be used to maximizethe electrical injection into the device. Other metals and metaldeposition techniques may be used.

Block 1707 represents the step of chemical mechanical polish. The backside of the transparent substrate may be thinned at this point forbetter thermal extraction and ease of die separation.

Block 1708 represents the step of die separation. The die separation maybe performed by a scribe-and-break process, by a laser separation or asawing process.

The thick GEMM (504) (1204) in this embodiment is greater than (greaterthan approximately) 150 nm in thickness such that it takes on its bulkreflective properties. The thickness of the GEMM is epitaxially grownbelow the crystalline relaxation critical thickness.

Processing Steps of Fifth, Sixth, Seventh and Eighth Embodiments

The exemplary processing for the fifth, sixth, seventh and eighthembodiments are similar to the previous first, second, third, fourthembodiments, respectively. The epitaxial structure is presented in TableI.

To be clear, in the fifth embodiment, the new n-GaN layer contact may bemade of metal materials that are highly reflective and provide therequired ohmic operation.

In the sixth, seventh and eighth embodiments the transparent contact maynot be required since the n-GaN layer itself is conductive enough thatno current spreading layer is required. This more conductive n-GaN layerallows for the light exiting layer to be more easily modified by etchingand photolithography techniques so that features may be formed forbetter light extraction.

Those that are skilled in the art understand that there are severalprocessing options that may be changed. The important point of theprocessing steps it to allow the growth structure and optical cavityoperation to be implemented.

Possible Modifications and Variations

A.) In alternative embodiments, the epitaxial crystal may be grown onother substrates (501) (601), (501) (801), (501) (1001), (501) (1201)for example, SiC, GaN, ZnO, MgO, glass, Si, GaAs, AN, LiGaO₂, LiAlO₂,NdGaO₃, ScAlMgO₄, Ca₈La₂(PO₄)₆O₂. If the substrates are opaque they maybe removed to allow light to exit during device operation.

B.) In alternative embodiments, the III-nitride buffer layer (502)(602), (502) (802), (502) (1002), (502) (1202) may thicker or thinnerthan 2 μm or may be doped to form electronic carriers.

C.) In alternative embodiments, the first deposited doped layer (503)(603), (503) (803), (503) (1003), (503) (1203) may be omitted if theGEMM (504) (604), (504) (804), (504) (1004), (504) (1204) is thickenough to sustain current flow for the device and if electricalterminals are connected to the GEMM (504) (604), (504) (804), (504)(1004), (504) (1204). (I.e., Bulk HfN is ˜30× more conductive thanmoderately doped bulk n-GaN). It may be desired to keep this first dopedlayer for fabrication yields during the etch steps (1402), (1502),(1603), (1703).

D.) In embodiments one, two, three and four the p-GaN layer may have atunnel junction and n-GaN layer added such that the new n-GaN layer ispositioned such as to increase the conductivity and current spreadingacross the LED.

E.) The removal of the substrate (opaque or transparent) to access thecrucial device epitaxial structure (partially transparent, partiallyreflective GEMM layer)/GaN/Active Region/GaN/(non epitaxial Metalmirror) in embodiment one or five may be another option formanufacturing, or device performance.

F.) The GEMM layer may be assisted with an epitaxial or nonepitaxial DBRwith a phase matching layer positioned between the GEMM layer and theDBR.

G.) The thickness of the GEMM layer may be thicker or thinner in theembodiments presented. It is possible to modify these thicknesses togain similar results from the embodiments. The thickness modification“window” can be large in various situations and should not be regardedas a major modification. What is important to these embodiments is theoperation and the tailoring of the cavity effects explained herein.

H.) The GEMM may be used to promote stimulated emission and photonrecycling in the device, and thereby increase the quantum efficiency ofthe device.

CONCLUSION

Those skilled in the art will appreciate that other modifications can bemade without departing from the spirit of the innovative conceptdescribed herein. It is not intended that the scope of the invention belimited to the specific embodiments illustrated and described in theembodiments.

1. A light-emitting device comprising: a first mirror having a firstface and an opposite second face and having a lattice constant, whereinthe first mirror includes a metal; and an epitaxial structure, whereinthe epitaxial structure covers at least a portion of the first face ofthe first mirror and has a lattice constant that substantially matchesthe lattice constant of the first mirror, the epitaxial structure havingan active region configured to emit light, wherein the active region isseparated by a first distance from the first face of the first mirrorsuch that the active region is located at or substantially at anantinode of a first standing optical wave produced by interference oflight emitted from the active region with light reflected by the firstmirror to form at least one extraction mode.
 2. A light emittercomprising: a) a first mirror that is an epitaxially grown metal mirror;b) a second mirror; c) an active region epitaxially grown such that theactive region is positioned at or close to, at least, one antinodebetween the first mirror and the second mirror.
 3. An epitaxy growthstructure comprising: a) a crystalline III-nitride layer; and b) planarspecular grown epitaxial metal mirror layer grown over the crystallineIII-nitride layer.